Logging apparatus



Aug. 1, 1961 w. E. MoTT LOGGING APPARATUS Filed April 2e, 195e w y 4 mr uw w m m u m 4 4 2 0 .,0 A a 9 @maa u f W n! f w P, P a n E uw 1w M w1 M United States Patent O 2,994,775 LOGGlNG APPARATUS William E. Mott, OHara Township, Allegheny County,

Pa., assignor to Gulf Research & Development Company, Pittsburgh, Pa., a corporation of Delaware Filed Apr. 26, 1956, Ser. No. 580,834 8 Claims. ('Cl. Z50-84.5)

This invention relates to new and useful improvements in borehole logging apparatus and especially to radiation sources thereof of the type wherein ions are accelerated to strike a target containing nuclei :reactive with the accelerated ions to produce the desired radiations. More particularly, this invention relates to such an apparatus wherein a substantial improvement towards achieving a constant radiation Output is obtained by maintaining a pressure differential between the ion accelerating portion of the apparatus and the ion forming portion of the apparatus in favor of the latter, and also by so controlling the pressure within the ion forming portion of the apparatus that ions are formed at a substantially constant rate.

The instant invention is related tothe same general class of subject matter as that disclosed in two other similarly assigned applications of mine led concurrently with this application, one of such other applications being U.S. Serial No. 580,833, tiled April 26, 1956, and entitled Borehole Logging, and the other of such -applications being U.S. Serial No. 580,906,`led April 26, 1956, and

entitled Stabilized Borehole Logging.

Broadly, the present invention has to do with an ion 4source and means responsive to the magnitudeof ionization current passing between the anode and the cathode of the ion source to control means that regulates the rate at which material to be ionized is to be introduced into the ion source. In general, the control is such as to vary the rate `at which the material to be ionized is introduced into the ion source in a sense reverse to the Sense of variations in magnitude of the ionization current. Stated otherwise, the rate at which material to be ionized is to be introduced into the ion source varies as an inverse function of the magnitude of the ionization or the ion current.

inasmuch as the magnitude of .the ionization current is a function of the pressure within the ion source, regulating the rate of introduction of material .to be ionized into the ion source in the manner defined in the preceding paragraph results in relative stability of the ionization current as well as of the rate at which ions are produced in the ion source. The 'invention also involves maintaining the pressure differential between the accelerator and the ion source in favor of the latter by means of the aforementioned introduction of material to be ionized into the ion source 4together with a pump for substantially evacuating the accelerator in such an arrangement that the source of penetrating radiation `can be contained within a housing adapted to be lowered within a borehole.

The invention also entails an` arrangement of a source of penetrating radiation of the character `above specified together with a radiation detector such that they can both be contained within a housing adapted to be lowered within a borehole, and also such `that the `target omponent of the source of penetrating radiation can, if desired, be placed in proximity to each other thatgis limited substantially solely by the amount of shielding material that it can be desired to interpose therebetween." 1

Yet another aspect of the subject invention involves the the following description of a preferredembodiment of ice the invention taken together with the accompanying drawings illustrative thereof, wherein:

FIGURE l is a diagrammatic illustration of the manner of use of the preferred embodiment of the invention;

FIGURE 2 is a diagrammatic illustration of the disposition of the elements of lthe source of penetrating radiation and the radiation detector; and

FIGURE 3 is a schematic representation of the electrical system associated with the elements of the source of penetrating radiation.

Referring to the drawings and to FIGURE 1 in particular, the numeral 10 designates a borehole which can be cased or uncased. The numeral 12 designates a housing supported by a cable 14, of the type that includes provision lfor an electrical conduit means 16 therein (see FIGURE 2), for vertical movement within the borehole.

The supporting cable 14 passes over a supporting pulley 18 and is wound upon a reel 20 above the earths surface 22. Means, not shown, are provided `for driving and braking the reel 2t) for raising and lowering the housing 12 in a conventional manner.

The interior of the housing is lshown in schematic detail in FIGURE 2. Suitably supported within the housing 12 is a vacuum tight casing 24 which is preferably metallic, and which like the housing 12 is constituted of a material that is substantially transparent to the penetrating radiation that is produced by elements within the casing 24 which are to be presently described. inasmuch as the expression penetrating radiation, as used in this specication and in the accompanying claims is meant to include neutrons as well as gamma rays or the combination of the ltwo, the casing 24 can be conveniently fabricated of aluminum, and the housing 12 of steel. Such materials are sufiiciently transparent to the penetrating radiations and afford sufficient structural strengths for their respec tive purposes.

Disposed in spaced relation within the casing 24, and preferably resiliently supported therein by means not shown to protect against mechanical shock damage is a hollow cylindrical container 26 formed of electrical insulative material such as glass that is closed at its opposite ends.

The container 26 encloses anion source designated generally at 28; means designated generally at 29 for introducing a material to be ionized to the ion source 28; an laccelerator designated generally at 32; a target 34; and a pump 36 preferably of the ionic type for maintaining -a high vacuum within the accelerator 32.

The ion source 28 comprises cathodes 38 and 40, and` a hollow cylindrical anode 42. The cathode 38 is formed with a plurality of openings 44 therethrough (see FIG- URE 3) so that material to be ionized can pass into the ion source 28. The cathode 38 is suitably supported upon the inner surface of the container 26 and can be insulated therefrom by a material having a high dielectric strength as shown. The cathode 40 is supported upon the inner surface of the container 26 by a vacuum tight sealing material 46, preferably having a high dielectric strength. Also, the cathode 40 is provided with a small central opening 48 that constitutes a restricted exit for the ion source 28 (see FIGURE 3) by means of which ions pass from the ion source 28 to the accelerator 32. The anode` 42 has its opposite ends spaced from the cathodes 38 and 40 and is supported upon the inner surface of thereontainer 26 as shown at 50 by a material preferably having a high dielectric strength..V The cathodes 38 and 40 are preferably formed of or include magnetized material` so of occurrence of ionizing events- The means 29 for introducing a material to be ionized into the ion source 28 includes -a vacuum-tight partition 52, which can be formed of glass or metal, adjacent the lower end of the container 26 so as toi define a Y reservoir 54 for material to be ionized. The means 29` for introducing material into the ion source 28 includes valve means 30 provided for regulating the rate at Which a gaseous material under an atmosphere or more pressure within the reservoir 54 is allowed to pass into the ion source 28. Conventional means, not shown, are provided for charging the reservoir 54. Although any type of Valve means 30 subject to electrical control for regulating minute gaseous flow rates can be employed, such as small spring-pressed needle valve controllably opened by an electric solenoid, the preferred form of valve means is the combination of a palladium thimble 56 sealed vacuumtight in an opening in the partition 52, and an electrical heating element 58 for heating the thimble 56 for the reason that the reservoir 54 will customarily contain gaseous hydrogen or one of its isotopes deuterium or tri- Y into the ion source 28 at a rate dependent upon the temperature of the palladium thimble 56 as controlled by the rate at which electrical energy is supplied to the electrical heating element 58.

The accelerator 32V comprises in its preferred form a `probe or focusing electrode 60 and accelerating electrodes 62 and 64. The latter mentioned electrodes are hollow cylindrical sleeves, while the probe electrode 60' is a hollow cylindrical sleeve that is necked down and provided with a small central` opening 66 (see FIGURE 3) adjacent the cathode 40. All of the electrodes 60, 62 and 64 are sealingly supported upon the, inner surface of the container 26, as shown, preferably by a material` having a high dielectric strength.

The target 34 is mounted upon one side of the inner surface of the Ycontainer 26 by a support 68, as shown, such support preferably being formed of a material having a high dielectric strength. The targetV 34- is arranged to be bombarded by ions accelerated by the accelerator 32,

preferably `in a` diffused manner, and includes atomic nuclei that undergo a nuclear reaction with the bambarding ions to produce the penetrating radiation, which it will be recalledvcan be either gamma rays or neutrons. Where it is desired that the penetrating radiation be highenergy neutrons, `the bombardingions can be deutrons andthe target 34 contain` nuclei of tritium, or alternatively, the bombarding ionscan be tritons and the target 3,4 include nuclei of deuterium. With either of such arrangements, ,where theV bombardingions have Vbeen ac.- celerated through a, potential difference of. about `20 or more kilovolts (higher accelerating potentials being 'preerred), a nuclear reaction occurs at the target` 34 pro ductive of neutrons having about 14 mev.

Where lowerenergyV neutrons Yare desired, the bombarding `ions can ,be deutrons accelerated through a potential difference of about 2 0 or more'kilovolts (higher accelerating potentials being preferred) and the target.34 contain nuclei of deuterium, in which case neutrons of about 2.5 mev. are producedjat the target.

. Where `the target 34 is to contain either; the nucleiof tritium or the nuclei-of deuteriuma the target can conven` ientlycomprise a` plate of tungsten coated or platedwith zirconium onthe side 'adjacent the accelerator 32nwith tritium-vor deuterium, as the case may be,`adsoibed in the zirconium; i

Y electrical system.- associated with the previously described.

Where it is desired that the penetrating radiation be gamma rays, many combinations of types of bombarding ions and types of nuclei to be included in the target will occur to those skilled in the art. For example, the bornbarding ions can be protons and the target include nuclei such as 3Li'7, 9F19, 6G12, or 6G13. Higher accelerating potentials are required to cause gamma-ray producing reactions with these combinations, than are required for theV previously mentioned neutron producing reactions. Accordingly, an appropriate selection of high-voltage source for the accelerator must be made in view of the fact that for maximum yield protons must be accelerated to energies on the orderof about 450 kilovolts to react with 3Li7 to produce 17 mev. gamma rays; 350 kilovolts to react with 9F19 to produce 6 mev. gamma rays; 450` kilovolts to react with C12 to produce 2 mev. gamma rays; and 560 kilovolts to react with C13 to produce 8 mev. gamma rays. Also, the target can include nuclei of 3Be9 to produce gamma rays of various energies up to about 7 mev. where an accelerating potential of about l megavolt or better is available. Lesser accelerating potentials or proton energies can of course be used in such combinations with reduced gamma-ray yield.

The pump 36 is for the purpose of maintaining a Very low pressure in the accelerator 32, say on the order of 10-5 to l()F6 mm. Hg. Though several well-known types of pumps are capable of maintaining such a high vacuum, the stringent space limitations inherent in borehole logging apparatus make it vastly preferable that the pump 30 be of the conventional ionic type. The schematically illustrated ionic pump 36 is suticiently small in size and comprises an electricalheating element for vaporizing substances such as zirconium, titanium, etc., thereon, which on condensating on adjacent surfaces, presents a large surface area that strongly adsorbs isotopes of hydrogen. Should the atmosphere within the accelerator 32 contain appreciable amounts of gases unsuited to the use of an ionic pump, as Where the bombarding ionshave a mass number in excess of three, resort rnust be made to some other type of high-vacuum pump that possesses the `requisite small physical size.n

VThe container 26 is provided with side openings at 72 and 74 so that the pump 36 can not only directly evacuate the interior of the accelerator 32, but also directly Vevacuate the space between the cathode 40 and the probe 6l). It is Within the province of the invention to provide an additional pump, not shown, for vthe latter purpose. Attention is now directed to FIGURE 3 Wherem the electrical elements is schematically illustrated.

A high voltage supply 76 having aV sufciently high voltage output for accelerating ions to the energy necessary to undergo the nuclear reaction productive of the desired penetrating radiation is provided which can convenientlyV b e of the Vari de Graaff or Cockcroft-Walton types, which supply -76 has its negative terminal 77 grounded as at 78 and is connected by a lead 80 to the electrode 64.v The electrode 62 and the target 34 are adjustably tapped by leads 82 and 84 to the positive side of the supply76, the arrangement being such that the target 341 has a potential sufficiently positive with respect to the electrode 64 `as .to` retard bombardment of the latter by electrons emitted by the target 34 during operation.

source is provided, which -has its negative terminal 87 Vconnected to the positiveterminal 88 of the supply 76.

by a lead 89. The cathode itl and the probe electrode 60 are adjustably tapped to the positive side of the supply V86 by leadsV 90 and 92,`respectively, with the cathode 4t) current-responsive device is -to control the rate at A further high voltage supply 86 for energizing theion` which electrical energy is supplied to the electrical heating element 58' for the palladium thimble 56 through leads 102 and 104. The arrangement is such that whenever the current flowing through the lead 96 falls below a predetermined value, the rate of supply of electrical energy to the heating element 58 is increased, and conversely, whenever the current through the lead 96 rises above said predetermined value,'the rate of supply of electrical energy to the heating element 58 is decreased. The current-responsive device 100 can be a conventional servo system adapted to function as above speciiied wherein the output of the sensing element thereof is responsive to variations of the electrical current flowing in lead 9'6 to control by means of an electr-ic motor the supply of electrical energy to the heating element 58 of the thimble 56. For example, though many other forms of servo systems or equivalents thereof will readily occur to those trained in this field, the current responsive device 100 can be such as that illustrated in FIGURE 2 of U.S. Patent No. 2,735,943 issued February 2l, 1956, to Wright et al., wherein the motor 72 thereof is arranged to drive a variable rheostat controlling the`supply of electrical energy to the heating element 58.

The electrical potential established between the anode 42 and the cathodes 38 and 40 is adjusted to be a substantially constant value in the range of say about 200 to about 5000'volts, and the previously mentioned predetermined current in the lead 96 -is that amount of current which will flow through the lead 96 when the electrical potential between the anode 42 and the cathodes 38 and 40 has been xed and the pressure within the ion source has been fixed at a value on the order of 103 to 10-4 mm. Hg.

It will -be appreciated by those skilled in the art that at pressures on the `order lof 10-3 to l()-4 mm. Hg within the ion source 28, and with a substantially fixed potential difference established between the anode 42 and the cathodes 38 and 40, the value of the ionization current flowing in the ion source 23 as represented by the current owing in the lead 96 is substantially a linear function of the pressure within the ion source 28. This, of course, is the principle upon `which ionization gauges are based. Accordingly, the above described function of the current responsive device 100, the heating element 58, and the palladium thirnble 56 is such as to maintain the pressure within the ion source 28 substantially constant. Maintenance of a substantially constant pressure within the ion source 28 assures a substantially constant rate of supply of ions to the accelerator 32. This in turn contributes materially to the attainment of a reasonably constant liux of penetrating radiation output of the target 34.

`A source of electrical energy 106 is provided for the ionic pump 36 and is `connected thereto by leads 108 and 110, the latter being grounded as shown.

As shown in FIGURE 2, the voltage supplies 76 and 86, the current responsive `device 100, and the source 106 are suitably mounted within the housing 112 below the container 26. As will be seen presently this arrangement places the high voltage equipment at a position remote from radiation detection apparatus subsequently to be described. If desired, the source 106 can be mounted in the housing 12 :above the casing 24, as the latter operates at a near ground potential.

Disposed within the housing 12 above the container 26 is a radiation detector 114 which can be of any desired type` to detect radiations entering the borehole 10 from the earth formations 115 as a consequence, however indirect, of the latter being subjected to penetrating radiation from the target 34.` For example, irrespective of the nature of the penetrating radiation produced bythe target 34, the radiation detector 114 can be the combination of a scintillation phosphor preferentially sensitive to either gamma-rays. or neutrons and a photomultiplier tube; or as a further example, the detector 114 can be a` 6 proportioned counter of either the gamma-ray or neutron detecting types. Also the detector `114 can be surrounded with any desired combination of gamma-ray shielding, neutron moderator, neutron absorber, or the like to effect any desired filtering or shielding action so as to obtain detection selectivity and/ or directivity.

Means are also provided for recording the output of the radiation detector 114 or any selected portion of such output with respect to the depth of the housing 12 within the borehole 10. A large number of such means are known which can be used for this purpose, the selection of the specific means to be employed being primarily dependent upon the nature of the information sought. By way of example, the radiation detector 114 is a combination of scintillation phosphor sensitive to gamma-rays and a photomultiplier tube with the output of the detector 114 being fed by as indicated at 116 to an amplifier `118. The output of the amplifier 118 is in turn fed to a pulse height analyzer 122 as indicated at 120. The output of the pulse-height analyzer 122 is fed by means indicated as the electric conduit means 16 within the cable 14 and a pickup circuit indicated at 126 connected to the reel 20, to a combination counting-rate meter and recorder shown at 128, the latter being operatively connected as indicated at 130 to the pulley 18 so as to obtain a record versus depth. If desired, the pulse-height analyzer 122 can be removed from the housing 12 and included in the surface equipment.

Means, not shown, of conventional character are provided for supplying electric energy through the electric conduit means -16 within the cable 14 to the elements 76, 86, 100, 106, 114, 118, and 122 within the housing 12. If desired, some of such elements can be supplied elecltrical energization by means of batteries, not shown, disposed with the housing 12.

In order to limit the extent to which penetrating radiation produced by the target 34 can directly reach the radiation detector 1,14, shielding means 132 can be disposed within the housing 1|2 intermediate the target 34 and the radiation detector 114. The character of the shielding means 32 Will be evident to those skilled in the art, lead being conventional for use as gamma-ray shielding, and al neutron moderator such as paraiiin backed by cadmium being conventional for use as neutron shielding. Both of such types of shielding can be combined to be effective shielding for a combination of both types of penetrating radiation produced by the target 34.

In view of the preceding detailed description of the apparatus, the overall operation thereof will be readily apparent. IFor purposes of describing the overall operation of the apparatus, it will be assumed that the same is being employed to irradiate the -earth formations 115 with neutrons of about 14 meV. Accordingly, the target 34 includes nuclei of tritium and the reservoir 54 contains deuterium gas. Deuterium ions are fed from the ion source 28 to the accelerator 32 at a substantially constant rate by reason of the previously described control of the pressure in the ion source 28 in response to the current in the lead 96. The deuterium ions are then accelerated `by the electrodes 62 and 64 to a suiiicient velocity to undergo a nuclear reaction with the tritium nuclei included in the target 34, such reaction being productive of 14 meV. neutrons. The neutrons thus produced proceed outwardly from the target 34 into the earth formations 115. 'Radiation of a preselected character, say gamma rays having 6 mev. energy, entering the borehole 10 from v the earth formations 115 as a consequence, however indirect, of the latter being subjected to irradiation by 14 mev. neutrons from the target 34 are detected and recorded versus depth by the elements 114, 118, 122, and

small horizontal cross section of space available in any borehole logging apparatus. Also, linear acceleration permits the target to be placed in relatively close proximity to the radiation detector Where such close spacing is desired.

The outstanding advantage of the described mode of maintaining a constant pressure within the ion source will be readily appreciated in View of the stability therefor afforded both as to rate of ion supply to the accelerator and the output of penetrating radiation. Obviously, such mode of pressure stabilization is vastly superior to the crude method heretofore employed of merely allowing gas to leak into an ion source. Also, the beneficial cooperation between themeans for introducing Vgas into the ion source and the ionic pump to maintain a pressure differential between the accelerator and the ion source in favor of the latter will be appreciated, as the arrangement enhances ion formation within the ion source, and increases the likelihood that an ion supplied to the accelerator will reach the target.

The illustrated preferred embodiment of the invention is subject to numerous variations without departing from the spirit of the invention. Exemplary of such variations would be, where an intermittent production of penetrating radiations is not objectionable or desired, the application of well known microwave pulsing techniques to the accelerator 32 with suitable accelerator electrode modification can be readily accomplished, as will be understood. Another variation would consist in the employment of conventional means for periodically interrupting energization of the ion source 28 where pulsing of the output of penetrating radiation is desi-red.

Also, encompassed within the scope of the invention is the preconditioning of the electrodes of the ion source and the accelerator. For example, Where all of such electrodes have zirconium surfaces this can be accomplished by desorbing all such electrodes vunder high vacuum at a temperature of about 500 to about 600 C., preferably while maintaining an electric discharge between the electrodes of the ion source. Thereafter the temperature of the electrodes of only the ion source are cooled to about 270 C., the discharge discontinued,

and the ion source is flooded with hydrogen or the isotope thereof subsequently to be ionized while the pressure is maintained at as low a value as possible in the accelera-V tor. After the electrodes of' the ion source have had sutlcient time to adsorb essentially an equilibrium quantity of gas, the pressure within the ion source and the accelerator is reduced, the electrodes of the-ion source are allowed to cool to normal temperature, and subsequently (after maximum desorption) the electrodes of the accelerator are also allowed to cool to normal temperature. Such preconditioning of the electrodes will the ion source and the detector, and means for maintaining a pressure differential between the ion source and the accelerator, said means comprising a reservoir for materialto be ionized, a diffusion barrier interposed between said reservoir and said ion source, electrical heating means adapted to heat said diiusion barrier, control means, means connecting said control means to at least one of said electrodes of said ion source, means connecting said control means to said electrical heating means, said control means being adapted to increase the electrical energy supplied to said electrical heating means in response to a decrease of ion current to the electrode to which said control means is connected, and a pump connected to said accelerator and adapted to reduce the pressure within said accelerator.

2. In borehole logging apparatus, an elongated housing adapted to be lowered in a borehole, a source of penetrating radiation and also a radiation detector disposed in the housing, said source of penetrating radiation comprising a linear ion accelerator for accelerating ions lengthwise of the housing and a target arranged to be bombarded by ions accelerated by said accelerator, said target including atomic nuclei reactive with bombarding ions to produce the penetrating radiation,.an ion source having an anode and a cathode and having a restricted exit communicating with the accelerator for supplying ions thereto, and means for maintaining a pressure differential between the ion source and the accelerator, said means comprising a reservoir for material to be ionized, Va diffusion barrier interposed between said reservoir and said ion source, electrically heating means adapted to heat said diffusion barrier, control means, means connecting said control means to said anode of said ion source, means connecting said control means to said electrical heating means, said control means being adapted to increase the electrical energy supplied to said electrical heating means in response to a decrease of ion current to said anode, and a pump connected to said accelerator and adapted Vto reduce the pressure within said accelerator.

3. In borehole logging apparatus, an elongated hous- Y ing adapted to be lowered in a borehole, a source of penetrating radiation and also a radiation detector'disposed in the housing, said source of penetrating radiation comprising a linear ion accelerator for accelerating ions lengthwise of the housing and a target arranged to` be bombarded by ions accelerated by the accelerator, said p target including atomic nuclei reactive with bombarding assist in maintaining the previously mentioned desired" pressure diiferential. Y

The detailed description of the preferred embodiment of the invention has been for the purpose of conveying a full and complete understanding of the principles involved, `and no inference as to the scope of the invention should `be derived ,'therefrom, but rather such scope should be ascertained upon reference `to the appended claims. f

l. In borehole logging apparatus, an elongated housing adapted to` be lowered Vin a borehole, a source of penetrating radiation-and also a radiation Vdetector disposed bombarded by ions accelerated by said accelerator, said target includingt atomic nuclei reactiverwith bombardinggions to .produce the penetratingy radiation, an ion Y source having electric discharge electrodes"and'having L a restricted exit communicatingwith the accelerator for i supplying ions thereto, said target beingV disposed between diffusion barrier interposed between said reservoir and said chamber, electrical heating means adapted to` heat said diffusion barrier, control means, means connecting said control means to said anode of said ion source, means connecting said control means to said electrical heating means, said control means being adapted to increase the electrical energy supplied to said electrical heating means in response to a decrease of ion current v to said anode, and an ionic pump connected'to said accelerator and adapted to reduce the pressure Within said accelerator.

4. ln `borehole logging apparatus, an elongated housingl adaptedto be lowered" in a borehole, a source of penetrating radiation and also a5 radiation detector dis-` posed in lthe housingsaid source of penetrating'radiation comprising alinear-ion accelerator for accelerating ions lengthwise of the housing and artargetarranged tollbe bombarded by ions accelerated by the accelerator, said target` including atomic nuclei reactive with bombarding `ions to .produce the penetrating radiation, ,and an` ion 'source having a restricted exit communicating with the accelerator for supplying ions thereto, said ion source comprising a casing having a chamber therein, an anode and a cathode in said chamber, means establishing la substantially constant electrical potential between said anode and said cathode, a reservoir for material to be ionized, a diiusion barrier interposed between said reservoir and said chamber, electrical heating means adapted to heat said diffusion barrier, control means, means connecting said control means to said anode of said ion source, means connecting said control means to said electrical heating means, said control means being adapted to increase the electrical energy supplied to said electrical heating means whenever the ion current to said anode falls below a desired value, and an ionic pump connected to said accelerator and adapted to reduce the pressure within said accelerator- 5. A pressure-stabilized ion source comprising a casing having a chamber therein, an anode and a cathode in said chamber, an electric circuit including said anode and said cathode and means -for establishing a substantially constant electrical potential between said anode and said cathode, a reservoir for material to be ionized, a ditusion barrier interposed between said reservoir and said chamber, electrical heating means adapted to heat said diffusion barrier, control means, means connecting said control means to said electric circuit, means connecting said control means to said electrical heating means, said control means being adapted to vary the electrical energy supplied to said heating means in response to the current in said electric circuit.

6. A pressure-stabilized ion source comprising a casing having a chamber therein, an anode and a cathode in in said chamber, means for establishing a substantially constant electrical potential between said anode and said cathode, a reservoir for material to be ionized, a diffusion barrier interposed between said reservoir and said chamber, electrical heating means adapted to heat said diiusion barrier, control means, means connecting said control means to said anode, means connecting control means to said electrical heating means, said control means being adapted to increase the electrical energy supplied to said electrical heating means in response to a decrease of ion current to said anode.

7. A pressure-stabilized ion source comprising a casing having a chamber therein, an anode and a cathode in said chamber, means Afor establishing a substantially constant electrical potential between said anode and said cathode, a reservoir =for material to be ionized, a diffusion barrier interposed between said `reservoir and said chamber, electrical heating means adapted to heat said dii'usion barrier, control means, means connecting said control means to said anode, means connecting said control means to said electrical heating means, said control means being adapted to vary the electrical energy supplied to said heating means in response to the ion current to said anode.

8. A pressure-stabilized ion source comprising a casing having a chamber therein, an anode and a cathode in said chamber, means for establishing a substantially constant electrical potential between said anode and said cathode, a reservoir for material to be ionized, a dilusion barrier interposed between said reservoir and said chamber, electrical heating means adapted to heat said diiusion barrier, control means, means connecting said control means to said anode, means connecting said control means to said electrical heating means, said control means being adapted to increase the electrical energy suplplied to said electrical heating means whenever the ion current to said anode falls below a desired value.

References Cited in the le of this patent UNITED STATES PATENTS 2,240,914 Schutze May 6, 1941 2,489,436 Salisbury Nov. 29, 1949 2,733,347 De Liban Ian. 3l, 1956 2,733,348 Lawton et `al Jan. 31, 1956 2,735,019 Dewan et al. Feb. 17, 1956 2,735,943 Wright et al. Feb. 21, 1956 2,745,970 Dewan May 15, 1956 2,769,096 Frey Oct. 30, 1956 2,789,229 Lawrence Apr. 16, 1957 2,792,500 Burk May 14, 1957 2,817,032 Batteau Dec. 17, 1957 2,842,695 Goodman July 8, 1958 2,894,136 Reinecke July 7, 1959 2,901,628 Lamb Aug. 25, 1959 2,908,823 Ely Oct. 13, 1959 2,914,677 Arnold Nov. 24, 1,959 

